1. Field of the Invention
This invention relates to methods for utilizing geothermal resources to produce energy, and in particular to methods of introducing scale inhibitors and flocculent needed for smooth operation of a geothermal resource.
2. State of the Art
Geothermal fluids contain vast amounts of thermal energy, which can be used to produce electrical power. In one process for producing electrical power, a naturally pressurized, hot, substantially liquid geothermal brine is recovered from a subterranean reservoir, then suddenly depressurized, or, as termed herein, flashed. This produces steam from the brine while cooling the liquid brine. The cooled geothermal brine is processed further and then typically injected into the ground to replenish the aquifer and prevent ground subsidence.
The hot, highly saline brine removed from the ground is saturated or nearly saturated in many ions and minerals. As the brine is flashed, many of these ions and minerals precipitate out of the remaining liquid solution. Some minerals, typically silicon-containing compounds--particularly silica and silicate-containing minerals, such as iron silicate--form small particles that precipitate slowly. The slow precipitation means that these already precipitating minerals form solids that plug the downstream fluid handling and injection apparatus. Therefore, it is greatly preferred to force these slowly precipitating solids out of the liquid before injection using a flocculent added to the brine in a clarification process. Featherstone U.S. Pat. No. 4,874,529, which Patent is hereby incorporated in full by reference, discusses the problem of slowly precipitating fine particulates and describes the solution of adding a flocculating agent to a two zone clarification process having a primary clarification zone and a secondary clarification zone as the solution to the problem.
However, not all minerals precipitate from the brine solution slowly. Alkaline earth salts (for example, barium sulfate, also known as barite, and calcium fluoride) are among the minerals frequently found in significant concentrations in geothermal brines. Alkaline earth salts tend to precipitate fairly quickly both before the clarification steps, as well as later in the disposal process, even in the injection wells. During the flashing step alkaline earth sulfates (for example, barium sulfate) start to precipitate and continue precipitating down-stream during further handling of the brine as it continues to cool. Barium sulfate presents an annoying problem in many geothermal systems since it starts to precipitate when the cooling brine is still as hot as 340.degree. F., a temperature produced early on in the energy production process, triggering the early precipitation of barium sulfate.
Now, the precipitation of barium sulfate has been implicated in a more serious precipitation problem. Recently, it has been noticed that the crystalline material precipitated at some geothermal sites becomes more radioactive as the site ages. The radioactivity, contained in naturally occurring radioactive materials, or NORMs, is believed to result from tapping into new subterranean formations containing trace amounts of a radioactive alkaline earth sulfate--radium sulfate--which apparently coprecipitates with the chemically similar barium sulfate. It is important to reduce the levels of NORMs in the material that precipitates during the clarification process on the processing equipment to as low a level as possible and inject as much of the NORMs as possible into the earth in the brine stream injected. A solution to this problem is the use of dispersants and crystal structure modifiers to prevent the alkaline earth salts from precipitating. This solution is described in U.S. patent application Ser. No. 717,527, filed Jun. 19, 1991, now allowed, and incorporated herein in full by reference.
In view of the foregoing, it can be seen that geothermal brines contain dissolved components that often cause processing problems because they contain unwanted components that are best left in the solution for subsequent disposal by injection into the earth while also containing unwanted components that are best removed as quickly as possible from the aqueous stream to avoid scaling and contamination of the processing equipment. The NORMs, which are associated with barium sulfate, contaminate the scale formed with radioactive components. However, even though the precipitation of NORMs from the brine stream is to be avoided, the precipitation of silicon-containing components, including silica and silicates, is required for smooth process operation. Consequently, the precipitation of the silicon containing components is induced in at least one clarification vessel by the addition of a flocculating agent.